Multiple layer paper containing pigmented pulp and method of making



Patented June 3, 1952 vMULTIPLE LAYER PAPER CONTAINING PIG- MENTED PULPAND METHOD OF MAKING William Lutton Craig, Westport, Conn., assignor toR. T. Vanderbilt Company, Inc., New York, N. Y., a corporation of NewYork No Drawing. Application April 30, 1946,

Serial No. 666,199

Claims. (01. 92-3) paper or board is commonly carried out in cylindermachines having a number of cylinder vats and cylinder molds, eachseparately supplied with the desired stock, and each forming a layer,with the layers combined to form the final sheet of multi-ply paper orboard, the number of lay-' ers depending upon the number of cylindervats and molds in use. The outside layers of the composite or multi-plypaper or board are formed from stock from the first and last vats andmolds and are called liners; and the intermediate layers are calledfillers, and are commonly made of less expensive stock.

In such cylinder machines the stocks are supplied from the stuff cheststo regulating boxes and from the regulating boxes to the cylinder vats,each containing a cylinder having a cylindrical screen on which a layerof partially formed Wet paper is produced, which is transferred to afelt. The successive layers of partially formed Wet paper from thecylinder molds form a composite or multi-ply series of layers, which arecarried with the felt between rolls to squeeze out Water and to compactthe layers, after which the sheet passes through press rolls to dryingcylinders and to calenders where a size may be applied. This method offorming laminated or multi-layer paper or board, and one form ofapparatus therefor, are described, for example, in vol. V of TheManufacture of Pulp and Paper (2nd Edn.) section 1, pages 251-264(McGraw- Hill Book Company). Such composite multilayer paper or boardcan similarly be produced in multi-Fourdrinier and other machines.

The improved process of the present invention can advantageously becarried out in such cylinder machines to give an improved coated boardor sheet without subsequent coating of the finished sheet or board in acoating machine.

According to the present invention, the composite sheet or board isprovided on one side or on both sides with a surface coating layer orliner in the form of a specially prepared pigment-fiber layer or fibrouspigment of highly pigmented, modified cellulosic fibers, or a layercontaining such highly pigmented modified cellulosic fibers admixed withuntreated fibers, which layer or liner, when sized,'gives an improvedcoated board or sheet on a paper-making machine and without subsequentmachine coating of the dried board or sheet.

The present invention is based upon the discovery that a portion of thecellulosic fiber stock (chemical, semi-chemical, or mechanical pulp)used in makingpaper, including cellulose fibers (chemical pulp) such askraft, sulfite, soda pulp, etc, and particularly groundwood(ligno-cellulose) fibers (mechanical pulp) can be treated in the form ofa suspension in water or pulp with calcium chloride in the form of astrong solution to modify the cellulose or the ligno-cellulose and thatthe so modified cellulosic fibers can then be treated with a solution ofa soluble salt cellulose fibers such as chemical p'ulp (white pulp,sulfate pulp, soda pulp, etc.) as well as semi-chemical pulp andgroundwood fibers (mechanical pulp), as well as mixtures of such fibersor pulps.

The cellulosic fibers are advantageously treated in the form of arelatively concentrated pulp or stock. Thus, stocks containing up to 4%or 5% or more of fibers (dry basis) are advantageously treated, and thetreatment is also applicable to much more concentrated'stocks at a stockconsistency as high as 13% or higher, where higher concentrations areavailable.

The stock treated may be bleached or unbleached stock and may be stockwhich has been beaten in abeater, particularly in the case of long fiberchemical pulp or unbeaten disintegrated stock.

The treatment of the fibers in suspension in strong calcium chloridesolution of sufficient strength to modify the cellulose or ligno-cellu-I about a'profound change in the nature and apcalcium chloride combineswitht hecellul'ose to form complex products, or it may.;be the-,caleiu nchloride exerts a modifying or hydrating efiect upon the cellulosemolecules. Whatever the i; 1-

theoretical explanation of the action of thecalcium chloride on thecellulose maybe; it brings pearance of the cellulose fibers. Similarly,;when H semi-chemical pulp or groundwood fibers are similarly, treatedwith calcium chloridethe action. is tobring abouta profound changein thenature ds-appearancee -m sh s- After thi' .treatmntWith calcium chloridethe stoclg-is treated with .a' solution of a. soluble salt. suchassodiumcarbonatel(sodaiash) I or' sodium bor =1, t e e.lv g'., m'th'elform l of. 'a 10%..sol1itioz1' Lin suflicient amount to react withthe calcium..chloride-and precipitate an insoluble pigment, e; g.,.calcium carbonate; .or calcium, .bo'rate in and onthelflinodifiedjfibe'r'ai. The amount ofv added solu ble saltfis,.equaltoI-orsomewhat more; or less. than'tliearnbu whi'ch suificientto react with" r all lot =thelcalcium chloride used. iWitli cellulose.fibers 'thegfamountl; of. soluble salt isl'adv'antageouslyflsli'ghtly,excess .of that required to react wit1 .a11',. z t e calcium chloride;but with grouri godlfibers' .care [should ingeneral ibe'takenQto.ayoidianyexcessof the alkali salt which 7 would darken orotherwise 'afiect the stock; and. 40 with suc toicki 'thecamum chlorideand. sodium ;7 carbonat' are; us'edflin .exajct' reacting proportionsgsli'gh tl'xcess. .of 'calciuins'chl'oride to I avoidanyvexcessflgfisodium carbonate;

'i'hejqre ii ins'itr a ed nd ipi mi nted. sioc i 4 highly: hydrated and;has 1ittle str'er'igth,";or rela'- tivel'y', l m'uch 1essj.strength Ythan the; untr d"? stock; but itfcarriesfthe precipitated. pigment; distb ited. Within .thih rat d' fi slwfii e p merit is 'aIsQpreci'pitated'on the hydratedifib'ers dh t e-W t i .whic the fibers iare sus-f ,i--..4'.. T V e 'spe i lly r atede d d fid1a dp s1 nt d stoc s hish1y;pie.en edI ..i.An'ian'1.ounit of piem ntq lb eemhine with thj' O k which Iis c demb yii 'exq s's o "the wei ht of .t g stock treated (dry basis).Thus cellulose fibers" can bem d fied and .pi m n edt eivea' p p t on Qpi nent" isto fd y ba i uri'tgo aroundtwotopne 1r more; GI'OlllfldWOOdstock can be pigmentedii'tdia greater extent, up to." aroundf i partsoffpigment 'to 1 of fiber (dry;

basis) or even higher.;

Theresulting stockfcontainingthe modified and p m ted fiber w thit epiement prec pie tated in and on jan'd" around the fibers, is used inthe ,improyed process; of thepresentjjinven-p' tion, either alone or.admixed with, untreated; stock, informing the coating or liner layer.of thecompositemulti -layer paper or board. I The stock, after;treatment with; the1so1uble I 7 sodium salt, will contain e. g; sodiumchloride in" solution; When 1 this" modified and j pigmented 1 stool; isused without'jadrnixture' of untreatedf stockjinmaging 'the coating ;orlin n layer. it

'at least 50 %2"- andadvantageously-nonsiderably I may not be necessaryto remove the sodium chloride, as most of the sodium chloride will beremoved in the cylinder machine in the white water, and most of theremainder will be squeezed out by the pressure of the squeezing andpress rolls....,It..is sometimes advantage u h w ig remove the s'odium;chloride solution by washing the treated stock, or to remove the greaterportion of the sodium chloride solution and replace it with fresh water,particularly where the stock isfii'niired witliuntr'eate'd stock and themixed stock- -is-us'ed "for"forming the coating or liner layer. Thewashing. of the treated stock can be carried jout by dec'antati'on or bywashing in pulp was, with. -replacement of the sodium chloride solutionwith iresh water, care being taken that theiwater drawn off is clear orrelatively clea r water to avoid or minimize loss of finelysuspendedpigment during the washing operation. I

The modified and pigmented stock before use.

' in thelast 'cylin'derwat of the cylindermachine;

may be [diluted to propelffconsistency; e.'. 'g. M around 1% of"cellulosic "fibers (dry basis) f or. f, somewhat less This 'stock,t inaddition to its content of modified cellulose, 'will containv a highf'percentage of precipitatedlpigment. someofthe'jf pigment may escape withthe white water but anf amount can be retained inth'e surface layer so3O that thisllayer may containeg. from 40 to u of precipitatedcalciunfi' carbonate distributed inf and on and around the inodifiedfibersg The. m fie d. p m nted. w ihv t admixture of untreated fibers,'can;advantage ously; be used iniorming the coating or l iner' layer ofthe oompositemulti-layer pulp or larc'iarcig This treated and pigmentedstock is highly hyi drated; and the liner layer'formed therefrom has;little strength or relatively muchless'streng'th .thanfa layer"formed'ir'o'm 'the untreated stock;-

or iiner' rbr:mmtamy pmxr and boardwhibh will haveincreasedfstrengtlr-andgwhich win also be' heavily pigmented;

VVHerethE' modi'fiEd and pigmented-stock :is-

thus-admixed withiintreated stock; varying proportions of the treatedand untreated stock's can be used; For making coated board directly onthe paper machine, the proportion of pigment inthe liner or coating-"layer-shoi 1ld,fin generalfbe above 5 0 I A highly pigmented coate'd"board can thus. be prodiicediwhich, aften s'izi t e calender-rolls;givsfa coated aboard v thepaper machine-which'tcanbereadily pi irite fIt is sometimes-desirable 'to' rdrm mu1ti p1y 'board 'with use-p gmentmati g. g' ar'oiiiij 3p to 40% pigrnm'it' -ifi' 'tli{ctiiatilig layer; amulti ply paper orboard wmemwhai 'si'Z'edIean' also'bereadily=prifitedHf Forfhighly' igmented coated board;-i t"i s desire"'ableand 'advant; eons; fwh'err untreated fibers are m xe w h h' i d fli mented-st c i to usejon y'a e t fel si ell r porti n 1. 1treatedistock e.fg.r 10% -or 2 0%j.,ofviohglJfib' percentages ofuntreated stock can be used, and

even when 50% of the stock is treated and pigmented and 50% isuntreated, a coating layer or liner can be formed which will containconsiderably in excess of 50% of pigment (as determined by ash) ascompared with the weight of fiber (dry basis). Thus, where 50% of thestock in the form of groundwood is treated to form a modified pigmentedstock containing 4 parts by weight of pigment to 1 part by weight offibers (dry basis) and when this treated stock is admixed with 50% ofuntreated cellulose stock, the weight of the pigment in the resultingstock and in the coating layer or liner formed therefrom may approximatetwice the weight of the fibers (dry basis). When 80% of the stock suchas groundwood is treated and admixed with 20%.of untreated long fiberchemical pulp, the pigment content of the coating layer or liner may bearound '75 to 80% of the weight of the coating layer or liner.

The process can be modified by adding organic binders, such as starch orcasein, etc., to the stock before it is used on the cylinder machine, e.g. an amount equal to around more or less, based on the carbonatepresent.

In the carrying out of the process on a multiple cylinder machine, thenew stock is used in the last cylinder vat, which produces the outerlayer or liner of the composite sheet or board. Various stock can beused in the other cylinder vats, such as are commonly used in makingmultiple layer sheets and board, such as less expensive stock made fromwaste paper or reclaimed kraft, etc., for the intermediate or fillerlayers. Except for the use of the new pigmented and modified cellulosicstock for the outer layer or liner, the process can be carried out muchthe same as that now used in making multi-layer sheets or board, andwith use of varying numbers of cyl; inder vats and cylinder molds toform composite sheets of from two to six or more layers, and with theouter layer or liner formed from the specially prepared and pigmentedstock.

t is one advantage of the present invention that a particularly highproportion of pigment can be incorporated in the stock used for theouter layer or liner and that the proportion of cellulosic fibers can berelatively small. A single sheet of paper made from such stock wouldhave little strength, but when used as an outer layer or liner for amulti-layer sheet or board, sufficient strength can be given by theother layers while the outer layer or liner is a highly pigmented layerwhich gives an improved pigmented finish to the sheet or board.

When the specially treated stock is formed into a sheet on the screen ofthe cylinder mold, the pigment tends to concentrate on the screen sideof the layer, giving a smoother and more finished appearance to thesheet than would the outer surface of the layer. The outer surface witha more fibrous appearance is the surface picked up by the felt so thatthe pigmented surface formed on the screen is the outer surface of thecomposite sheet or board.

With highly pigmented stock containing a higher proportion of pigmentthan of fiber, and particularly with two or three or four times as muchpigment as fiber, the dried composite sheet may contain so much pigmentthat it will tend to rub off unless protected. But by applying size tothis surface of the sheet in the calenders, the size will hold theexcess pigment and give a highly pigmented and sized surface with auniform coating that completely covers all fibers.

and gives a pigment face with a high proportion of finely dividedcalcium carbonate pigment.

Finished sheets or boards can thus be produced with a white surfacewhich can be printed, lacquered, etc.

The danger of removal of the surface pigment by dusting can also bereduced by adding binders to the stock before, it is applied to form thesurface layer or liner; and a further improved result can be obtained bysizing the sheet in the calenders in accordance with the common practiceof applying sizing to sheets during the calendering operation.

Where the treated and pigmented stock is admixed with untreated stock,the untreated stock may be stock which is prepared e. g. in a beater inthe ordinary way and which may be sized and prepared for use before thetreated stock is added. The stocks, are thoroughly admixed togetherbefore the composite stock goes to the cylinder vat, and the mixing isadvantageously carried out at a point close to the cylinder vat.

Rosin soap and alum are commonly added to the untreated stock forsizing. Where untreated stock is admixed with the treated and pigmentedstock, if the stocks after admixture are not sufficiently acid, afurther amount of alum is advantageously added at the time of mixing orbefore the admixed stocks go to thecylinder vat or is added to thecylinder vat. The untreated stock may thus be ordinary stock which isprepared in the beater and which may be sized and prepared before thespecially treated and pigmented stock is admixed therewith to form thecomposite pigmented stock for use in making the coating layer or linerof the multi-ply paper or board.

Where admixed untreated stock and treated pigmented stock are used, theportion. of stock treated may be either cellulose stock (chemical pulp)or semi-chemical pulp or groundwood stock. In many cases it is moreadvantageous to treat groundwood stock because of the increased amountof pigment which can be incorporated in itand retained by it, care beingtaken as above explained to avoid excess of alkali in the-treatmentwhich would tend to darken or otherwise affect the groundwood stocktreated. When untreated stock is admixed with the treated stock, theuntreated stock is advantageously cellulose stock such as chemical pulp,and particularly long fiber chemical pulp which will give increasedstrength to the coating layer or liner.

The invention will be further illustrated by the following specificexamples:

Example I.Sulfate kraft pulp from long-fibered wood is prepared in abeater with a content of around 4 to 5% of fibers (dry basis). The stockso prepared is placed in a separate tank provided with agitators, andsolid calcium chloride is added in amount sufiicient to form a solutioncontaining about 20% of calcium chloride. The stock is stirred to insurethorough distribution of the calcium chloride and to permit its actionon the fibers, and this is continued for a period of 5 to 10 minutes,but may be continued for a longer period. After the calcium chloride hasexerted its modifying action on the cellulose fibers, there is added asufficient amount of 10% sodium carbonate (soda ash) solution to reactwith thecalcium chloride. This reaction takes place in a period of about5 to 10 minutes with continued agitation, but the action can bepermitted to take place for a longer time. If the sodium chloride, is tobe'removed more-or less 1 7'1 completelyflbm th'e'astflckti' itiszmvasheds-or de -a cantedwith the -addition of; treshiwaterfito ei-i.place the salt solutioni 'Betoreuse in the cylinder machine; thes stockis diluted to proper consistency; Gaga; around1% i-5 of modifiedcellulose or a SOmGWhaULLIOWBI'iLpBZZ Ln' centage, and is-thencsuppliedato thesstufiiehest regulating box and cylinder avatforluse;in::mak+i. ing the-outer layer or liner of the compositeishetz';or board v The -multii cylinderz machine 'may hex-:10. otherwiseoperated in the -usual way=to iformatheze multi layer sheet or boardi-whichsiis -;.;pa'ssed ;a through the: squeeze and-pressure:ro'llsiandaoveren the dryers, and calenders, being givenz-c-aisizi-ngrs;treatmentby applying. a; starchisolutiomor a waf- L te'rprooi'size onthe 'calenders E$ample: II.+-2400 parts of :sgrounded-tipuip on: 2.5%fibercontent equivalent to=-60?,"parts=-=of:d fibers are 'pumped to atreatingtanlneandtze lmz parts of calcium: chloride ;.74-%" CaCl2)' areadded 120.2;

and-mixed thoroughly. and d-issolved and tlre -agi tation iscontinuedioraboutll) minute's to effect thorough treatment and gellingof the stock by the action of calciumchloride; A sodiunrearbonatesolution is made by dissolving tiofl poundsot swa 5;:

layer or liner makes it desirable to use a size in ash in water to forma solution :ofabout: 10-7 This solution is pumped into the=caloiumchloride t at p t b a ralmw djt mix-therewith, witlwcontinuediagitationg untilicomplete precipitation 1 has occurredozTheresulting treated stock-in =the form-bf:afibrous precipitate is'pumpedto storage' tanks oand hllowedt to settlel' without i agitation.

Afters. settling the-=top liquor- -(-sodiu'm chloride solution) isdrawn: off zand replaced with fresh water toxgive a treated:stoeleWhich;-afteimifllifibers The procedure. is otherwisethe sameas A tionstopropercconsistency; is used in the cylinde'r machine .for? formingetht:outer layeror linerof theacomposite sheet orboard "as-described'- inExample-IL 8Z2 thereto-1: talc-, size; alumi: andsstarchr. the" :proetportion-of 25:ipar ts oiitalc; 2.5? parts of resin size' (rosin -soap).3.5 parts ofchighfree rosiriisiZer; (rosinsoap cont'aining about 46Ifreevrosin)2 6 parts' -of:alum :and 6 parts of starchfor 240 partsoifimixed stock (dry basis). This treated-stock is-passedflfrom thebeater through the. Jordan, through the machine chestand-head box inlthescreens to the: sluice leading to the cylinder vat, and th'etreated-stock is admixed with the um-- treated stoek in. the slui'ce,the mixed stocks being then pumpedto theLcylin'der' vat.

Tlfaproportions'of stocks admixedare 15% 01 1 thestochivzdry .basis)which has been "specially-' treated to imodify. and pigment it, 7 and25% '-Jof th'e untreatedstock furnish; Alum is added-to keep the stockin the cylinder vat ac'id and: at a a. pH of-arQundBl-O 1106.2.

The. mixed-stock from the cylinder vatis used toiiform the-outerlayerlor liner. of a multi-ply paper sheet or board in a regular multicylin'der h machine, and the resulting composite paper or board ispassed from the cylinder' vats-through the: usual equipment includingcalender rolls.

The: porous and absorptive nature of the outer sizing the board Which isa non-penetrating size such as a-low viscosity; high- 'solids starch izecontaining addition agents which give a non- :penetr'ating size; V

ExampZcI-VJ-The sametreateotand untreated stocksare used as in ExampleIIL' but the -proportionszof treated and untreated stock are thesame,-that-is,' 50% of the fibers are modified and pigmented fibers'and- 50% are regular untreated that: of the preceding :example.-Although only half of -the stock-used in forming the outer layer orliner -is modified and pigmented stock, a high Examples =I anduIIdescribe-the use---o the '40 iproportionof pigment is includedin thecommodified andipigmentedstock withoutadmixture-1 of.untreatedtstockiormaking the coating la'yer or lin'er of thevmulti=plypaper or board;

Thefollowing examplesedescribthe also" of admixed i'itreatedxanctspigmented stook -with eun- -45 treated stools. 1

Example: III."- '-The-stobk-duinishwa portion-off whiohxisi untreatedanda Q0rtrinf0f' whioh istreated, is. made-sit 1 850 parts of"unbleached dong-5 fiber .sulfite i ipulpgs35ii 'parts offiyleafshavings eo 3 loitparts; of ozhard white stock (long fiber bleached-jsulfite) Y and SLOQOiiIpaItSKJf deinked fstoele idry basis). Thisstoolofurnish is prepared in*abeate;- in? the regular-manner a-t fi 7%consistency;

A portion of this stock-*iuT-n-ish *amourrting'$065 3,000 pounds ofmixed sto ck at-' 6-%"-consistency l-pounds 'of "fiber dry -'bas is)1 is 'placedjn" a washer in which it -'-=kept in circulation andf somewhatdiluted to permit'-;goodcirculation-. T" 1 this .1 diluted "stockisadded1-,000- pounds of'7 5'%-}"fi0 calcium? chloride 4750 poundsCaGI2Fiwhich -isi; dissolved: andall'owed to-"* reac t fo'r 15minutes;with continued circulation 700 poun'ds'of1soda: ash is dissloved in:water to --form -afi 1(I-%" solu' tion, and after the-reaction ofthe-calcium phi ride onlthe stockis 'completed the soda' ash solu; tionis: pumpedin with continued ;circul-a, tio1 d; of the-Strick andprecipitation of calcium: carbonate" 1 allowed to. take 1 place; thiscarbonate bmg pre V cipitatedswithinas Well as-=on and around' thefibers; i Afternthewprecipitatiorr ise completed; they stocks-is Washedlbyrdrawingoflthesodiunr-chlio ride;solution:and-meplaoingdt with-freshwater:

A separatez;portiomiofi the same. stock {Turn-15h pOSi-te stock,amounting to considerably more th'an=:50% of the weight of the linerlayer While inv the multi-ply paper or boarcl of Example III; a muchhigher proportion of pigment is-containew in the:outer--layer or linerof the composite sheet ori'boardit EzcampZe- V-InsteaJd of using treatedstock- 1 Which is a cellulose stoclnor largely a cellulose stock,= as=in Examples III and -I V, groundwood canbeitreatedto modify and: pigmentit, as in Example-:II, and the resulting modified-and pigmentedgroundwoodstock-i admixed with 'untreated cellulose stock-and theresulting mixture used toi'form the outer layer or liner ofthecompositepaper or board, accordingto the procedure followed in ExamplesIII-or IV.-

While the foregoing examples illustrate particular m'ethodsof treatmentandparticular pros portions of admixedtreated and pigmented stock and'untrea-ted stockgand'the-use-of treated-stock alone-,' for-forming theouterlayeror liner ofthe multi ply paper or board, it willbe -evidentthatvariations can be made -in' the amount of "pig ment -incorporated inthe tre'ated an'd'pigmented stoek and also in the proportions ofpigmented stock and untreated stock, where such mixtures areused.-Thus,- the outer-layer or "liner may be -made entirely- 0f thetreated-and-pigmented stockg as illustrated in Examples I andII, andthepropo'rtion of pigment'to fiber-(dry. basis) in'the treated' stool;may be considerably in ex.- 7, cess of'-5 0%="andmay amount 'to as muchas '4, times theaweight of thej'fiber or more:

I Where highlypigmcnted imodifieid". .fiberseare admixed withuntreatedifibera'the 'proportionof' 1 untreated fibers is'formany'purposes advantageously only a small percentage of the totalfibers, e. g.,-10% of long fiber chemical pulp admixed with 90% ofpigmented cellulose pulp or groundwood stock; or the proportion of.untreated stock may be higher, e. g. 20% or 25% or moreof the total orin some cases as muchas 50%, as illustrated in Example IV.

It is one advantage of the process of the present invention that it canbe carried out at room temperature throughout and with the use ofexisting equipment in a multi-cylinder machine, al though the tank orapparatus where the calcium chloride is added should be constructed ofmaterial which is not corroded by the calcium chloride solution.

It is another advantage of ,the invention that it enables afinished'carbonatewoated sheet to be produced directly on the. papermaking machine without subsequent treatment of the sheet or board bysubsequent machine coating.

While the invention has been described more particularly in connectionwith the use of a-multicylinder machine, the process can be carried outin a multi-Fourdrinier machine or in other machines for making laminatedor'multi-ply paper where separate wet layers are formedand joinedtogether to make the composite sheet or board.

The present invention enables a uniformly coated sheet to be cheaplyproduced with the coating formed as a part of the paper making operationand without subsequent coating.

The number of plies or layers of the multi-ply sheet or board can bevaried. For example, in making a sheet or board weighing about 50 poundsper thousand square feet, layers or plies may be used, with the outerpigmented layer or liner forming about pounds of the total weight perthousand square feet, or somewhat more or less, and with the pigmentedouter layer varying in calcium carbonate content from an amount equal toor somewhat less than the fiber content of this layer up to an amount 2or 3 or 4 times as large as the fiber content or even higher. Thefinished board, weighing around 50 pounds per thousand square feet, maythus contain from 5 to 10 pounds of the coating layer per thousandsquare feet, with the highly pigmented white surface coating finallysized to give a white finished surface which can be printed, lacqueredor otherwise finished when the board is used in making boxes, cartons,etc.

The new composite sheet or board is characterized and distinguished byits outer layer or liner of highly pigmented, modified cellulosic fiberscontaining pigment both in and on and around the fibers. Even with alarge excess of calcium carbonate pigment in the surface layer or liner,the application of non-penetrating starch size in the calender will holdany otherwise loose particles of pigment firmly bound in the surfacelayer or liner.

Even where highly pigmented coated sheets or board are not desired, buta filled surface board of lower pigment content, such board can readilybe produced by the use of admixed modified pigmented stock withuntreated stock to give a surface layer or liner containing e. g. around30 to 40% of pigment. Such a composite sheet, after sizing, givesdirectly on the paper machine a filled sheet which can be printed, etc.

With higher amounts of pigment, a coated sheet can be directly producedon the paper machine, so that it will have a hard surface which can beprinted, etc., without subsequent coating treatment.

It will thus be seen that the present invention provides an improvedprocess for making multiple layer sheets of paper and. board and givesa. resulting product which is an improved of increased strength andwhich will contain the untreated fibers together with the modified andpigmented fibers intimately admixed and combined therewith in the outersurface layer or liner. v

I claim:

1. The method of making multiple layer paper sheets and boardwhi'chcomprises subjecting an aqueous suspension of cellulosic fibers,at ordinary temperatures, to the action of a strong solution of calciumchloride until the calcium chloride brings about a gelatinization of thecellulosic fibers, adding a soluble salt which will react with thecalcium chloride to precipitate a calcium compound pigment in and on thegelatinized fibers, the amount of calcium chloride in the solution beingsufficient that the amount of precipitated calcium compound remaining inand on the fibers is in excess of the weight of the fibers, and applyingthe resulting stock to form the outer layer or liner in making amultiple layer sheet or board.

2. The method of making multiple layer paper sheets and board as setforth in claim 1 in which the cellulosic fibers treated are groundwoodfibers.

3. The method of making multiple layer paper sheets and board as setforth in claim 1 in which the cellulosic fibers treated arechemicallytreated fibers.

4. The method of" making multiple layer paper sheets and board as setforth in claim 1 in which sodium carbonate is added to react with thecalcium chloride to form a calcium carbonate pigment in and on thegelatinized fibers.

5. The method of making multiple layer paper sheets and board with ahighly pigmented outer surface layer or coating which comprisesapplying, in making the outer layer or liner of the multiple layer sheetor board, a pulp of highly pigmented and gelatinized cellulosic fiberscontaining an amount of finely-divided precipitated calcium compoundpigment in excess of the dry weight of the fibers, said pigmented fibersbeing produced from swollen, hydrated and gelatinized cellulosic fiberscontaining a soluble salt largely within the fibers by reaction ofanother soluble salt therewith to precipitate the calcium compoundpigment largely within the fibers as well as on said fibers.

6. The method according to claim 5 in which the highly pigmented andgelatinized cellulosic fibers are groundwood and carry an amount ofpigment more than twice the dry weight of the fibers.

'7. The method according to claim 5 in which the highly pigmented andgelatinized cellulosic fibers are chemically-treated fibers and carry anamount of pigment more than twice the dry weight of the fibers.

8. A composite multi-layer sheet of paper hav-

8. A COMPOSITE MULTI-LAYER SHEET OF PAPER HAVING AN OUTER OR SURFACECOATING LAYER FORMED OF HIGHLY PIGMENTED AND GELATINIZED CELLULOSICFIBERS CONTAINING AN AMOUNT OF FINELY-DIVIDED PRECIPITATED CALCIUMCOMPOUND PIGMENT IN EXCESS OF THE DRY WEIGHT OF THE FIBERS, SAIDGELATINIZED AND PIGMENTED FIBERS HAVING BEEN PRODUCED FROM SWOLLEN,HYDRATED AND GALATINIZED CELLULOSIC FIBERS CONTAINING A SOLUBLE SALTLARGELY WITHIN THE FIBERS BY REACTION OF ANOTHER SOLUBLE SALT THEREWITHTO FORM THE PRECIPITATED CALCIUM COMPOUND PIGMENT LARGELY WITHIN THEFIBERS AS WELL AS ON THE FIBERS.